The use of thick film conductors in hybrid microelectronic circuits is well known in the electronic field. Such materials are generally comprised of a dispersion of finely divided noble metal or metal alloy powders, with a mixture of metal oxide or metal oxide-forming powders and glasses with an organic vehicle to form a paste-like material. The consistency and rheology of the paste are adjusted to give suitable performance with the particular method of application such as screen printing, brushing, dipping, extrusion, spraying or syringe dispensing. Such pastes are generally applied to a suitable substrate material such as 96-99% alumina to give a patterned thick film conductor layer. The paste is generally dried at temperatures of 100.degree.-150.degree. C. to get rid of the volatile components and then the patterned conductor is fired, typically in a belt furnace, to burn out the non-volatile organics (e.g., ethyl cellulose, resins, rosins, thixotropic agents), and to sinter metal particles, thereby densifying the metal film.
There are several performance-related properties which the conductor paste and subsequently the fired conductor film must possess in order for the product to be commercially viable. It is often necessary to balance the chemistry and metallurgy of the thick film paste composition in order to obtain an acceptable level of overall performance, since the materials used to make the conductor can sometimes affect the performance in different ways.
The processing of the materials described herein is conventional, i.e., the belt furnaces, printers, drying equipment are standard and well known to those familiar to the art of electronic thick films. Furnaces which have generally been used are convection heated with heavy gauge resistance wire elements imbedded in ceramic refractory material.
Although the processing of the materials of the present invention may be considered "standard", the properties of the materials after processing are "state-of-the-art". For example, many of the advanced applications of today require a combination of performance properties which before now have been impossible to achieve with thick film materials of the past. These properties are:
thermal cycle adhesion PA1 thermal aged adhesion PA1 solder leach resistance PA1 soderability/solder acceptance PA1 conductivity.
All of these properties relate to the ease with which circuits can be manufactured and the reliability of the final circuits. For example, one of the most recent reliability requirements of thick film conductors in a wide range of applications pertains to the "thermal cycle adhesion" (TCA) or ability of a soldered thick film conductor to withstand repeated cyclings from low to high temperature. Particularly when solder is employed, these cyclings cause a rapid degradation of the thick film adhesion to the substrate material, ultimately leading to total loss of adhesion in the worst case. Thus, there is a great need for a way to obtain high thermal cycle adhesion values need for a way to obtain high thermal cycle adhesion values without losing any of the other equally important performance properties.